The present invention relates to an electric discharge machining power source for an electric discharge machine for machining a workpiece by using a capacitor discharge.
A conventional discharge circuit of an electric discharge machining power source for performing capacitor discharge is illustrated in FIG. 1. A pulse is supplied to a base q2 of a transistor Q2 as a switching element to repeat the ON/OFF operation of the transistor Q2, and a capacitor C0 is charged by a DC high-voltage source through a resistor R0. A charge voltage is applied between an electrode P and a workpiece W, and the discharge current from the capacitor C0 is generated as a spark between the electrode P and the workpiece W, thereby performing electric discharge machining. In this case, in order to stop discharge between the electrode and the workpiece and perform stable discharge, a transistor Q1 as a switching element is inserted in the discharge circuit, as shown in FIG. 1. When the capacitor C0 is completely charged, the transistor Q2 is turned off, and the transistor Q1 is turned on, thereby starting discharge. In this case, when the switching transistor Q1 is turned off during discharge, the switching transistor Q1 can be damaged by a surge voltage generated by an inductance inherent in the electric discharge machine. The switching transistor Q1 must be turned off after the discharge is completed. However, in the electric discharge machine, the capacitance of the capacitor C0 must vary in order to change a discharge time. The discharge time also changes in accordance with the inductance inherent in the electric discharge machine. For this reason, in order to protect the switching transistor Q1 from damage, it must be turned off within a maximum discharge time in view of the above conditions. However, machining efficiency is degraded by such a method of operation.